Genesis of Nucleation Sites Dictates the Fate of 2D MoS2 Growth

Scalable synthesis of two-dimensional (2D) transition metal dichalcogenides (TMDs) with atomic precision is critical for next-generation flexible electronics. Chemical vapor deposition is a primary strategy for the growth of 2D TMD films. The process, however, is difficult to control at large scales due to the strong interplay between time-dependent concentration gradients of the precursors, setup-specific fluid dynamics, and surface energetics. Here, we present a dual-channel reactor system that allows independent dynamic control over transition metal and chalcogen precursor dosages. This enables preprogrammed delivery of precursor partial pressures, effectively decoupling the chemical potentials governing nucleation and subsequent growth phases. This was demonstrated using molybdenum disulfide (MoS2) synthesis as an archetypical model system. By combining experiments and time-dependent fluid dynamics simulations, we have revealed that the genesis of nucleation sites, the local Mo/S ratio established during the fleeting initial nucleation event, decisively dictates the ultimate growth morphology. We demonstrate that near-stoichiometric S-rich conditions during both nucleation and growth stages favor 2D lateral expansion via heterogeneous surface reactions, whereas excessively S-rich conditions promote 3D growth through homogeneous gas-phase reactions. Furthermore, we show that the spatial limitations of traditional horizontal configurations, such as inherent concentration gradients and mass transport bottlenecks, are effectively bypassed using an optimized vertical substrate orientation. By thinning the boundary layer and stabilizing the chemical potential, this configuration shifts the rate-limiting step toward surface reaction kinetics. This dual approach of temporal precursor modulation and spatial substrate optimization provides a robust protocol for the deterministic synthesis of 2D TMDs.